Anticancer chemotherapy was set about progressing as choriocarcinoma was cured completely by using methotrexate. As of today, about 50 different kinds of anticancer drugs have been used and especially, choriocarcinoma, leukemia, Wilms' tumor, Ewing's sarcoma, rhabdomyoma, retinoblastoma, lymphoma, mycosis fungoides, testis tumor et cetera have been satisfactorily treated with those anticancer drugs.
Recently, knowledge about the development of cancer and the characteristics of tumor cells has been disclosed a lot and studies concerning the development of new anticancer drugs have followed. Most anticancer drugs show anticancer effect by suppressing the synthesis of nucleic acid of tumor cells or defunctioning the nucleic acid by directly combining with the nucleic acid. However, they have serious side effects such as bone marrow depression, gastrointestinal damage and lose hair since these anticancer drugs work not only tumor cells but also for normal cells.
The biggest problem of using anticancer drugs is that those drugs do not selectively work for only tumor cells. That is; anticancer drugs are working for every cells showing fast division or proliferation (bone marrow cells, epitherial cells of stomach and intestines, hair follicle cells, etc.), causing almost every cancer patients to be suffering from side effects such as bone marrow depression, gastrointestinal trouble, and lose hair, etc. Nevertheless, the anticancer drugs have therapeutic effect against cancer because tumor cells respond more sensitively than normal cells, so that more tumor cells are destroyed than normal cells, in addition, normal cells are regenerated faster than tumor cells. Meanwhile, besides the anticancer effect, those anticancer drugs also have anti-immune effect. Thus, it is another use of anticancer drugs to be provided to patients who need organ transplantation for the purpose of eliminating rejection symptoms after transplantation. But the danger of infection should be considered for cancer patients since those drugs drop immunity.
About 50 anticancer drugs have been widely used so far. These drugs are classified according to their reaction mechanism and components. Among them, adriamycin, commonly called doxorubicin, is highly effective for the treatment of malignant lymphoma, acute myeloid leukemia, soft tissue osteosarcoma, breast cancer, ovarian cancer, lung cancer, bronchial cancer, bladder cancer, digestive system cancer, etc. Although it is a very effective anticancer drug, it still presents such side effects as severe bone marrow depression, hypofunction of heart and kidney, and outflow of blood from blood vessels into tissues (N. Eng. J. Med., 1981, 305, 139).
Cancer chemotherapy is very limited because of the toxic side effects of anticancer drugs. As explained above, side effects results from the fact that the anticancer drugs used in chemotherapy lack efficient selectivity for tumor cells. To suppress the toxic side effects of the anticancer drugs to normal cells and to improve their efficiency toward malignant cells, lots of studies have been carried out. The preferable methods are using micelle or microsphere as a carrier of anticancer drug and conjugating anticancer drugs to polymeric carriers.
The first method that uses micelle or microsphere as a carrier of anticancer drug is to reduce side effects of cancer treatment by inserting anticancer drug into micelle or microsphere and letting it release slowly. When anticancer drug is administered separately, it works in a short period in large quantities, by which side effects are caused. This method is a good try to reduce those side effects by inducing slow release of the anticancer drug under the condition of enveloped in micelle or microsphere (Pharm. Res., 1983, 15, 1844).
The second method is to produce anticancer drug-polymer complex by combining the drug with polymer. Side effects are caused from the fact that the anticancer drugs used in the present cancer chemotherapy lack efficient selectivity for tumor cells. Thus, studies of conjugating anticancer drugs to polymeric carriers have been carried out as one promising approach to suppress the side effects of the anticancer drugs to normal cells and to improve their efficiency toward tumor cells. Expected advantageous features of this method are preferable tissue distribution of drug given by the character of the polymeric carrier, prolonged half-life of drug. in plasma, and controlled drug release from the polymeric carrier by adjustment of the chemical properties of the bond between the drug and the carrier.
Several kinds of polymers, naturally occurring and synthetic polymers have been studied as carriers of anticancer drugs. Among naturally occurring polymers, immunoglobulins are most widely used as the carrier due to their high specificity and wide applicability to many kinds of tumor cells. Utility of immunoglobuline as the polymeric carrier is, however, restricted by its chemical and physical properties. For example, modification of immunoglobulins by anticancer drugs often leads to precipitation due to hydrophobicity of the drugs. Furthermore, modification procedures are limited to ones performed in mild conditions to avoid denaturation of the immunoglobulins during modification.
Recently, the polymeric carrier of the drug can be freely designed using many kinds of synthetic polymers available today, and various organic reactions can be used to introduce drug to the synthetic polymeric carrier. From this point of view, several kinds of synthetic polymers have been investigated, such as poly(N-2-(hydroxypropyl)methacrylamide), poly(divinyl ether-co-maleic anhydride), poly(styrene-co-maleic anhydride), dextran, poly(ethylene glycol), poly(L-glutamic acid), poly(aspartic acid) and poly(L-lysine).
Using pathophysiological characteristics of tumor tissues with anticancer drug-polymer complex, cancer can be treated. Generally in tumor tissues, more blood vessels are generated than in normal tissues in order to get enough nutrition for the growth of tumor cells. The blood vessels in tumor tissues have bigger size than those in normal tissues but their structure is defective. Drainage through a lymphatic duct is also very limited comparing normal tissues. Therefore, polymers easily permeate into tumor tissues but hardly be excreted from tumor tissues. This specific phenomenon showed in tumor tissues is called enhanced permeability and retention (EPR) effect (Adv. Drug Deliv. Rev., 2000, 65, 271). As one of the treatments using anticancer drug-polymer complex, the attempt using N-(hygroxypropyl)methacrylamide (HPMA)-anticancer drug complex is under the phase II clinical trial (U.S. Pat. No. 5,037,883 (1991)).
The anticancer drug-polymer complex forming self-aggregates is expected to have a large diameter, as compared with unbound drug, which is a small molecule. The polymeric drug having ideal diameter is expected to circulate in the blood stream without embolization at capillaries, to escape from excretion in kidney, and to permeate into the target cells through blood vessels. And this self-aggregates form is expected to help protect the conjugated drug from enzymatic attack in plasma by concealing the conjugated drug with the polymer.
As a precursor of chitosan, chitin is a natural polymer comprising (1→4)-β -glycoside bond in which N-acethyl-D-glucosamine units are repeated and is generally found in outer coat of insects including invertebrate Crustacea and cell wall of fungi. Chitosan is a basic polysaccharide generated through N-deacethylation by treating chitin with the high concentration of alkali. Chitosan has been known to be superior to other synthetic polymers in cell adsorption capacity, biocompatibility, biodegradability and plasticity.
Thus, the present inventors have synthesized a novel anticancer drug-chitosan complex having strong points of micelle by making anticancer drug react with a polymer directly to form self-aggregates and making the anticancer drug be induced therein, which is different from the way of inserting anticancer drug into micelle. And, the present invention has been accomplished by confirming that the anticancer drug-chitosan complex can release the drug slowly and continuously, and can be controlled drug release by adjustment of the chemical properties of the bond between the drug and the chitosan, resulting in high selectivity against tumor tissues.